package main

import "fmt"

func climbStairs(n int) int {
	if n < 4 {
		return n
	}
	a, b := 2, 3
	r := 0
	for i := 4; i <= n; i++ {
		r = a + b
		a = b
		b = r
	}
	return r
}

func main() {
	r := 6
	a := climbStairs(r)
	fmt.Println(a)
}
